The present invention relates generally to data communications networks and more particularly relates to an ATM switch incorporating optical switching.
Recently, more and more reliance is being placed on data communication networks to carry increasing amounts of data. In a data communications network, data is transmitted from end to end in groups of bits which are called packets, frames, cells, messages, etc. depending on the type of data communication network. For example, Ethernet networks transport frames, X.25 and TCP/IP networks transport packets and ATM networks transport cells. Regardless of what the data unit is called, each data unit is defined as part of the complete message that the higher level software application desires to send from a source to a destination. Alternatively, the application may wish to send the data unit to multiple destinations.
Asynchronous Transfer Mode (ATM) originated as a telecommunication concept defined by the Comite Consulatif International Telegraphique et Telephonique (CCITT), now known as the International Telecommunications Union (ITU), and the American National Standards Institute (ANSI) for carrying user traffic on any User to Network Interface (UNI) and to facilitate multimedia networking between high speed devices at multi-megabit data rates. ATM is a method for transferring network traffic, including voice, video and data, at high speed. Using this connection oriented switched networking technology centered around a switch, a great number of virtual connections can be supported by multiple applications through the same physical connection. The switching technology enables bandwidth to be dedicated for each application, overcoming the problems that exist in a shared media networking technology, like Ethernet, Token Ring and Fiber Distributed Data Interface (FDDI). In addition, ATM allows different types of physical layer technology to share the same higher layerxe2x80x94the ATM layer.
More information on ATM networks can be found in the book xe2x80x9cATM: The New Paradigm for Internet, Intranet and Residential Broadband Services and Applications,xe2x80x9d Timothy Kwok, Prentice Hall, 1998.
ATM used very short, fixed length packets called cells. The first five bytes, called the header, of each cell contain the information necessary to deliver the cell to its destination. The cell header also provides the network with the ability to implement congestion control and traffic management mechanisms. The fixed length cells offer smaller and more predictable switching delays as cell switching is less complex than variable length packet switching and can be accomplished in hardware for many cells in parallel. The cell format also allows for multi-protocol transmissions. Since ATM is protocol transparent, the various protocols can be transported at the same time. With ATM, phone, fax, video, data and other information can be transported simultaneously.
ATM is a connection oriented transport service. To access the ATM network, a station requests a virtual circuit between itself and other end stations, using the signaling protocol to the ATM switch. ATM provides the User Network Interface (UNI) which is typically used to interconnect an ATM user with an ATM switch that is managed as part of the same network.
ATM is the enabling network technology for the high speed transmission of voice, data, video and multimedia information over communication links. Local Area Networks (LANs) based on ATM technology are becoming more and more popular due to the high speed and flexibility of ATM switching. ATM networks are typically configured in the shape of a star configuration with all the stations connected to a central switch or group of switches forming the network backbone. Currently, high speed electronic switches are used to route the ATM cells over the appropriate links. Although ATM is typically transmitted using optical communication links, the switches themselves are currently electronic based.
The benefits of optical communications are not utilized in current systems due to the frequent signal transformations from the optical domain to the electrical domain in order to provide the switching and routing functions.
In addition, the typical architecture of an ATM central switch includes a central ATM switch fabric and one or more interface (I/F) cards (modules). The interface modules incorporate the ATM cell processing, buffering and connection to the user interface which may be copper or optical fiber based. The central switch fabric includes the switch matrix and associated control functions thereof. The connection between the switch fabric and the interface modules is carried over a backplane printed circuit board (PCB). The connection is usually copper based and can be either parallel or serial.
There are numerous commercial vendors today that offer off the shelf building blocks for constructing such a switch. The main limitation, however, of this type of architecture is the throughput of the switch fabric which is currently limited to approximately 30 to 50 Gbps. The limitation is due to the limitations of current electronic switching technology. Another limitation is the connection over the backplane where the maximum bandwidth achievable today is limited to about 2.5 Gbps for each serial connection. Parallel connections can achieve much higher data rates but require large numbers of high speed serial connections aggregated together over the backplane PCB.
Using ATM network technology as an example, the current topology of high performance ATM local area networks (LANs) includes ATM core switches at the backbone and an edge device having an ATM downlink to the one or more core switches. When a connection is established between two edge devices, the traffic must pass through the ATM switches in the core. Therefore, in order to support all potential connections between all edge devices, the ATM switches at the core need to be non blocking. Non blocking ATM switches are difficult to develop and thus are much more expensive.
In addition to the disadvantage described above, the resulting network may be limited in bandwidth. When attempting to establish large numbers of connections from the edge device, there may be a need for faster downlink data rates. Depending on the number of connections and the throughput required for each connection, the downlink capacity may not be sufficient to meet the needs of users.
An additional disadvantage is the amount of physical wiring required to implement such a network. In practice, each edge device must be connected to the ATM core via physical wires (i.e., cables). When considering a typical office building there may be many wires installed in parallel. A separate cable from each edge device on each floor must be run down to the ATM core farm which typically is located in the basement. Wherever the switch core farm or server is located, cables must be run from the switch core farm to each edge device. The total length of the required cabling can be relatively very high and thus have an associated very high cost.
The cost may be even higher depending on the type and length of cabling used. For example, in ATM networks, it is common to run high speed fiber optic cable from the ATM switch core to all the edge devices in the network. Data rates may range from OC-3 155 Mbps to OC-12 622 Mbps on the optical fiber, for example. Note that each optical fiber used in the network carries only a single communication channel using a single wavelength of light. If it is desired to maintain several communications channels at one time, more than one optical fiber is required. Using prior art transmission techniques, each communication channel requires a separate optical fiber.
Today, most legacy local area networks utilize ATM technology in combination with Switched Ethernet or Token Ring network topologies. The existing switching technology enables each user on the network to have their own dedicated bandwidth, e.g., 10 Mbps or 100 Mbps, for their networked software applications. Each user is given network connectivity to the local switched hub, e.g., 100 Mbps for a Fast Ethernet network interface card (NIC). In typical office building environments, each floor is provided with one or more switched hubs that users are directly connected to. If the switched hub has 16 10 Mbps ports than it may potentially be forced to handle an aggregate data rate of 1,600 Mbps data rate from all the connected users.
Wave division multiplexing (WDM) technology enables the simultaneous transmission of multiple data channel connections on the same physical optical fiber. This is achieved by utilizing several different wavelengths on the same optical fiber at the same time.
Using this type of network, several data sources can be sent simultaneously into a WDM mux whereby each data source uses a different wavelength. The optical WDM mux functions to combine the different wavelengths into one optical transmission light beam. This optical light beam is transmitted onto the optical fiber using an optical transmitter. The fiber carries multiple connections simultaneously. The optical light beam reaches an optical receiver which outputs the light beam to a WDM demux. The WDM demus functions to split the optical light beam into the different wavelengths that were originally sent. The different wavelength outputs of the WDM demux are input to individual receivers which convert the light energy into electrical signals.
Currently, the major use of WDM technology is in Wide Area Network (WAN) applications. The majority of WANs already have a large installed base of optical fiber. The optical fiber installed in WANs typically carry very high data rate traffic on the order of many gigabits per second. In addition, the demand for bandwidth and capacity is growing at an explosive rate. Today""s WAN installations are being pushed to capacity in order to satisfy the demand for increasing levels of bandwidth.
Two different techniques can be used to transmit data at higher rates: (1) adding additional optical fibers or (2) to increase the rate of data at the edge devices on either end of the optical fiber. Both of these solutions are very costly: installing additional fiber optic cable is very costly and developing faster end equipment is difficult and expensive.
Currently available WDM technology, however, is a viable alternative to installing new fiber optic cable or upgrading the equipment on either end of the fiber. Using conventional WDM technology, several xe2x80x98slowxe2x80x99 conventional end devices can be connected to a WDM mux whereby several slower data sources are combined onto the same fiber and transmitted to the other end. At the far end of the fiber optic cable, the operation is reversed, i.e., the optical signal is optically WDM demuxed. Thus, WDM technology can be used as a bandwidth concentrator.
The present invention is an ATM switch which utilizes an all optical switching fabric to perform switching functions. The switch is based on fiber optics and Dense Wavelength Division Multiplexing (DWDM) technologies. The switch utilizes a centrally located optical switch fabric with multiple distributed interface cards. The optical switch fabric itself is substantially unlimited in bandwidth since the switching is performed using optical signals rather than electronic signals. Thus, a switch using such a switching fabric that utilized light can potentially reach data rates in the Terabit range. The switch utilizes fiber optic signal paths on the backplane to achieve substantially unlimited bandwidth from the interface card to the switching fabric. With currently available technology, each fiber optic link can carry traffic at data rates of 10 Gbps and up.
The switch module comprises an N X N passive star coupler which functions to pass the optical signal input present at any of its input ports to all of its output ports. Each output port has associated with it a unique wavelength. An optical filter set to the wavelength functions to block all wavelength except for the wavelength associated with that particular output port. The ingress portion of each receive/transmit interface module comprises a tunable optical transmitter which is set to the wavelength corresponding to the desired destination output port. Cells are transmitted to the switch module on that wavelength and only the optical receiver associated with the wavelength receives the optical signal. The other output ports filter the signal. In this fashion, ATM cells received are switched to any output port utilizing WDM optical processing.
Throughout this document the term wave division multiplexing (WDM) denotes using a single optical fiber to transmit several communications channels simultaneously whereby each channel transmits data utilizing a different wavelength of light. The term dense wavelength division multiplexing (DWDM) denotes WDM that utilizes several wavelengths of light that are relatively close to one another.
There is provided in accordance with the present invention an optical Asynchronous Transfer Mode (ATM) switch comprising a plurality of receive/transmit interface modules, each interface module adapted to generate and process ATM cells from signals received over a receive link, the interface module adapted to transmit signals over a transmit link in accordance with processed ATM cells to be transmitted, the interface module operative to determine a destination output port and a wavelength associated therewith, a switch module have N input ports and N output ports wherein one input port and one input port are associated with each interface module, the switch module operative to pass an optical signal present at any of its input ports to all of its output ports and wherein each interface module transmits ATM cells to the switch module utilizing a particular wavelength associated with the destination output port and wherein each interface module is adapted to filter all wavelengths of light except for the wavelength associated therewith.
The switch further comprises means for sensing the presence of optical signal at the wavelength to be transmitted to an input port on the switch module. The interface module comprises a receive interface for converting an optical signal received over the receive link to an electrical signal, an ATM layer cell processor for processing cells received from the receive link, a tunable optical transmitter for transmitting an optical signal at a wavelength configured therewith, a memory for storing one or more cell queues and a scheduler for determining the order in which queues are to transmit data to the switch module, the scheduler setting the tunable optical transmitter to the wavelength corresponding to the destination output port associated with the cell to be transmitted to the switch module.
The interface module comprises an optical filter set to a wavelength corresponding to the wavelength assigned to the output port on that particular interface module, an optical receiver for converting an optical signal to an electrical signal, an ATM Layer cell processor for processing cells to be transmitted over the transmit link and a transmit interface for converting an electrical signal to an optical signal for transmission over the transmit link.
The switch further comprises a tunable optical sense unit adapted to receive an optical signal from an output port on the switch, the tunable optical sense unit for detecting the presence of optical signal at a wavelength configurable therein. The switch module comprises a passive star N X N optical coupler.